Benefits of Marine Reserves - The Big Picture
As published in Dive NZ magazine, December2002/January 2003 issue
The world’s oceans are under stress from human activities, such as pollution and excessive harvesting, and in many areas are seriously degraded. Biodiversity - the richness of life - in the sea is suffering as a result.
Even in New Zealand there are major areas of concern. Despite the introduction of the world-acclaimed Quota Management System for fisheries, many of our fisheries are still well below the level required to support the maximum sustainable yield. Some fishing also leads to bycatch problems, loss of biodiversity within a fishery, and direct or indirect damage to benthic ecology.
Marine reserves are an insurance against ignorance and management mistakes. They do not replace other management systems, but are additional to, and supportive of them. Marine reserves preserve elements of biodiversity not adequately protected by current fisheries management, such as:
- Large experienced animals which can help "guide" younger ones.
- Genetic diversity within the large, old members of the population.
- Important social interactions within populations.
- Large old animals which produce more juveniles because they have lived longer and have been breeding for more years.
- Larger animals which, in one season, produce disproportionately more eggs than smaller animals, for example a 10-kilo fish may produce 10 times as many eggs as five 2-kilo fish.
- Other things we know nothing of.
How widespread are the impacts of fishing?
All of New Zealand's coastal waters have been impacted by fishing. No areas remain in their natural state.
There are many examples of direct damage to marine life by fishing methods, such as the destruction of giant corals and seafans on deep seamounts by trawling for orange roughy, bycatch of sealions and fur seals in squid and hoki trawl fisheries, bycatch of albatrosses in long-line fisheries, and entanglement of dolphins in set nets.
Indirect damage to marine ecology may be less obvious but can be widespread. For example, snapper and crayfish are important predators of kina (sea urchins). Left alone, they effectively control kina numbers. However, in Northland and the Bay of Plenty, overfishing of snapper and crayfish during the last 40 years has allowed kina numbers to increase unchecked. These grazers eat seaweeds, and expanded their occupation of the rocky bottom at the expense of the kelp forests which provide shelter for many fish and invertebrates. What we are now left with are large barren areas with abundant kina, where once there were lush kelp forests.
The diagram shows what has happened. On the right is a natural area with high biodiversity before fishing. Natural numbers and sizes of snapper and crayfish keep kina numbers low and allow the kelp cover to remain generally continuous. On the left is a heavily fished area. Snapper and crayfish are depleted, and kina have expanded and eaten the kelp, seriously altering the ecological balance. This is typical of most of the coastline from East Cape to North Cape.
The good news is that marine life recovers when left undisturbed. If you leave the system alone, there will be a gradual shift back to the right, to the natural balance of kina, kelp, snapper and crayfish. Marine reserves allow this to happen, and then provide a reservoir of breeding individuals to help re-stock depleted areas elsewhere.
A network of marine reserves.
Because so much of the country's underwater environment is altered by human activities, we need to protect representative bits so that they can recover. Ultimately they would become examples of how the coast would be in its natural state, and provide reservoirs of breeding stock to help repopulate the depleted areas. We need a network of areas protected so that marine plants and animals can, at some stage in their life cycle, move from one protected area to another.
No-one knows the "best" distance between the reserves in the network, and it will be different for different species, but the principle is to create a network that is self-sustaining.
If we look at a map of the east coast of Northland and the northern Bay of Plenty, there are several marine reserves in place from Mayor Island in the south to the Poor Knights in the north, with a few small inner Hauraki Gulf examples close to Auckland. These marine reserves are beginning to form a network, but with several obvious gaps. If we look at the rest of the country, there is not even the beginning of a network, just a few isolated reserves separated by hundreds of kilometres.
Does size matter?
There is no "best" size for a marine reserve. For some organisms a very small reserve may be enough to protect a local population. For others, such as species that move around a lot or migrate, a very large marine reserve may be required to be effective. Some of the more mobile species are known to take up temporary residence within marine reserves. The best solution is to provide several reserves of different sizes.
Whatever the size, people fishing for snapper and crayfish just outside marine reserve boundaries impact on numbers in the reserve. The result is that these species are generally less abundant close to the edges of reserves than nearer the middle. This "edge effect" can be quite significant, especially in small reserves. Research at Leigh suggests that snapper and crayfish show reduced numbers as much as two kilometres inside the reserve, with higher numbers only near the middle of the 5-kilometre long marine reserve. A bigger reserve minimises this problem, as the affected edges become a smaller proportion of a large reserve.
Larger reserves also have the potential to protect a wider range of habitats, and hence more types of organisms, thus enhancing biodiversity.
How much is enough?
The New Zealand Biodiversity Strategy 2000 is a Government Policy document which aims to protect 10% of New Zealand's marine environment by 2010 to effectively conserve marine biodiversity. At the moment over 99% of our coastal seas are open to fishing. Reducing this to 90% is not an unreasonable goal.
What happens when an area is protected?
- All direct effects of fishing are eliminated.
- The area is recolonised by juveniles settling and adults migrating.
- Fishes accumulate and grow bigger.
- Rare and more vulnerable species accumulate.
- Biodiversity increases.
- Kelp forest recover as Kina numbers drop.
- Larger and more fishes result in greater production of eggs and larvae which are dispersed by currents, benefitting biodiversity and fishing in areas even a long distance from the reserve.
- Fishing improves near the reserve.
- Numbers of people enjoying the area increases.
- Complete recovery can take 25 years or more, although many benefits occur within five years.